Update Chinese monthly review docs

[Pigbibi]

Summary

• align README.zh-CN monthly AI review docs with the current CryptoCodexAuditBridge default path
• clarify legacy dual-AI API review is optional fallback only
• document that GCP_PROJECT_ID/GCS_BUCKET are vars-only, not secret fallback values
• add a regression assertion for the Chinese README defaults

Verification

• ~/.cache/crypto-codex-audit/python-venv/bin/python -m unittest tests.test_monthly_publish_workflow_config
• ~/.cache/crypto-codex-audit/python-venv/bin/python -m unittest tests.test_release_contract tests.test_release_status_summary tests.test_monthly_review_briefing tests.test_monthly_build_telegram tests.test_monthly_report_bundle tests.test_monthly_publish_workflow_config
• ~/.cache/crypto-codex-audit/python-venv/bin/python -m compileall -q scripts src tests